Nitrous oxide, commonly known as laughing gas, is a chemical compound with the formula N2O. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. Nitrous oxide is a greenhouse gas, accounting for around 6% of the estimated heating effect of greenhouse gases in the atmosphere. According to 2006 data from the United States Environmental Protection Agency, industrial sources make up only about 20% of human caused industrial sources. Other human activity may account for 30%; tropical soils and oceanic release account for 70%. Human-caused nitrogen loading to river networks is a potentially important source of nitrous oxide emission to the atmosphere which may have been severely underestimated. It happens via a microbial process called denitrification, which converts nitrates to nitrous oxide and other gases.

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When summed across the globe, scientists report this week in the journal Proceedings of the National Academy of Sciences (PNAS), that river and stream networks are the source of at least 10 percent of human-caused nitrous oxide emissions to the atmosphere.

That's three times the amount estimated by the Intergovernmental Panel on Climate Change (IPCC).

Rates of nitrous oxide production via denitrification in small streams increase with nitrate concentrations.

Denitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. Many Nitrogen oxide types may evolve at the same time. In terms of the general nitrogen cycle, denitrification completes the cycle by returning N2 to the atmosphere. The process is performed primarily by heterotrophic bacteria.

"Human activities, including fossil fuel combustion and intensive agriculture, have increased the availability of nitrogen in the environment," says Jake Beaulieu of the University of Notre Dame and the U.S. Environmental Protection Agency in Cincinnati, Ohio, and lead author of the PNAS paper.

Nitrates are highly soluble and are present in many fertilizers and are easily transported by water flow into streams and from there all thew way to the sea. Denitrification is just the natural way to covert excess nitrates (food to the bacteria) back into Nitrogen to begin the cycle all over again.

"Much of this nitrogen is transported into river and stream networks," he says, "where it may be converted to nitrous oxide, a potent greenhouse gas, via the activity of microbes."

Beaulieu and co-authors measured nitrous oxide production rates from denitrification in 72 streams draining multiple land-use types across the United States.

"This multi-site experiment clearly establishes streams and rivers as important sources of nitrous oxide," says Henry Gholz, program director in NSF's Division of Environmental Biology, which funded the research.

Atmospheric nitrous oxide concentration has increased by some 20 percent over the past century, and continues to rise at a rate of about 0.2 to 0.3 percent per year.

The global warming potential of nitrous oxide is estimated as 300-fold greater than just carbon dioxide.

While more than 99 percent of denitrified nitrogen in streams is converted to the inert gas nitrogen rather than nitrous oxide, river networks are still the leading sources of global nitrous oxide emissions, according to the new results.